Most transmission channels, such as those composed of public telephone lines, exhibit the same types of characteristics and differ from each other only in the degree of significance of the various factors involved. Transmission channels are usually characterized by amplitude and envelope delay characteristics which vary as a function of frequency. The amplitude characteristic gives the attenuation in decibels for each frequency that lies within the passband of the channel. The envelope delay for a given frequency is the propagation time of that frequency through the transmission channel. The envelope delay characteristic represents the relative propagation times in milliseconds of the frequencies that lie within the passband of the channel. For more complete definitions, reference may be made to the Bell System Data Communications Technical Reference entitled, "Transmission Parameters Affecting Voiceband Data Transmission--Description of Parameters," PUB 41008, July 1974. The amplitude and envelope delay characteristics lie within given profiles that define the quality of the transmission channel or, as it is sometimes referred to, the "quality of the line." The technique that is the most widely used for measuring the envelope delay characteristic is described in the Bell System Data Communications Technical Reference entitled, "Transmission Parameters Affecting Voiceband Data Transmission--Measuring Techniques," PUB 41009, January 1972. Briefly, this technique consists of measuring the slope of the phase-frequency characteristic of the line for each frequency lying within the passband of the line. This technique is both complex and time-consuming and requires special test equipment which the telephone companies alone may use to test their lines. Also, the user of a line is generally unaware of its exact envelope delay characteristic and merely knows the profile, as indicated by the telephone company, within which this characteristic lies. It has been found that a knowledge of both the profile of the envelope delay characteristic and the slope of that characteristic is sufficient for the purposes of many applications.
The envelope delay characteristic is an important feature of a transmission channel. Since the envelope delay varies with frequency, a signal transmitted over a channel undergoes a certain amount of distortion which combines with the amplitude distortion to create an interference between successive pulses that is known as intersymbol interference. Theoretically, if the precise envelope delay and amplitude characteristic were known, the effects of intersymbol interference could be eliminated by constructing a filter whose transfer function would be the inverse of that of the channel. In practice, however, these characteristics are not precisely known. In addition, the characteristics vary slowly with time.
The effects of intersymbol interference can be eliminated, or at least reduced, by means of a well-known automatic and adaptive device called an equalizer. An adaptive equalizer is a complex device whose cost significantly affects the overall cost of the receiver into which it is incorporated.
In data transmission systems in which the signaling rate is equal to or lower than 2400 bits per second, the effects of intersymbol interference are less detrimental to transmission and provision is normally made for a single fixed equalizer called a compromise equalizer which is much less expensive than an adaptive equalizer. A set of fixed equalizers having different transfer functions may be substituted for the single fixed equalizer. An initialization procedure permits selection of the particular equalizer to be used for a given transmission. The most widely used initialization procedure consists of sequentially connecting the various equalizers in the transmission path and then selecting the particular equalizer which gives the best results. This procedure is time-consuming and, consequently, increases the cost of the transmission.